Remove Zygote tests from DynamicPPL/Turing

[yebai]

#806 explicitly states that the only officially supported ADs for DynamicPPL are ForwardDiff/ReverseDiff/Mooncake, so we could, in principle, remove Zygote from DynamicPPL/Turing tests.

[penelopeysm]

There's some Tracker stuff in DynamicPPL too 😅